IP Multimedia (IPMM) is an example of a service that provides a dynamic combination of voice, video, messaging, data, etc, within the same session. By growing the numbers of basic applications and the media that it is possible to combine, the number of services offered to the end users will grow, and the inter-personal communication experience will be enriched. This will lead to a new generation of personalised, rich multimedia communication services, e.g. peer-to-peer multimedia communication, IPTV etc.
These services can be based on the IP Multimedia Subsystem (IMS) architecture, which is the technology defined by the Third Generation Partnership Project (3GPP) to provide IP Multimedia services over mobile communication networks (3GPP TS 22.228, TS 23.228, TS 24.229, TS 29.228, TS 29.229, TS 29.328 and TS 29.329 Releases 5 to 7).
The IMS makes use of the Session Initiation Protocol (SIP) to set up and control calls or sessions between user terminals (or user terminals and application servers). The
Session Description Protocol (SDP), carried by SIP signalling, is used to describe and negotiate the media components of the session. FIG. 1 illustrates schematically how the IMS fits into the mobile network architecture in the case of a 3GPP PS access domain.
IMS networks can be accessed using access networks, typically wireless access networks but also fixed line access networks. Referring to FIG. 2A, when a user wishes to access a network using a terminal (shown as terminal 1 in FIG. 2A) connected to a fixed line, the user may first be authenticated in an access network using Network Attachment Sub-System (NASS) (ETSI TS 282 004 v1.1.1). A Connectivity Session Location and Repository Functions (CLF) is a NASS entity that allocates an IP address to the terminal, and authenticates and authorises the user. The NASS can also be used to configure preferences in the access network, depending on a user profile stored by the NASS. If the user subsequently registers with an IMS network, the IMS network makes use of the IP address allocated to the terminal in order to establish a session between the user and, for example, a peer user or other media source.
Referring to FIG. 2B, a problem arises if the original terminal (shown as terminal 1 in FIG. 2B), for any reason, loses its connection with the access network whilst an IMS session is active. In this case, terminal 1 loses its registration in the access network, without closing down its IMS registration. IP addresses are allocated dynamically by the CLF, and so a subsequent terminal (shown as terminal 2 in FIG. 2B) that connects to the access network may be allocated the same IP address that was allocated to the original terminal. In this case, IP packets relating to an IMS session established by the original terminal will be sent to the subsequent terminal, giving the subsequent terminal unauthorised access to that session.